The present invention relates to a system for enciphering and deciphering data for transmission between a transmitter and a receiver, where the terms encipher and decipher are synonymous with encrypt and decrypt respectively. In particular, it relates to a system wherein clear data texts are enciphered at the transmitter end of the system and deciphered at the receiver end. Enciphering and deciphering are effected by the same set of instructions, the instructions being executed by cipher computers responsive to a cipher program stored at the transmitter and at the receiver. That is, the program of the cipher computers is determined by a secret key stored at the transmitter and at the receiver.
Such a system is disclosed, for example, in German Offenlegungsschrift No. 2,943,726, a large number of these systems being on the market. In general, they include cipher computers that operate according to special algorithms and are realized by digital circuits in the form of large-scale integrated circuits (LSI). A disadvantage of such systems is that only those having the same cipher algorithm can work together.
With the introduction of increasingly extensive communications networks, including international networks, there is an increasing demand for enciphered information exchanges. While standardization of the cipher algorithms would be one solution to the problem, it appears unlikely that each standardization will be realized for some time, one reason being that ciphering devices employing different kinds of cipher algorithms have already been introduced into commercial use.
It is an object of the present invention to provide a cipher system which permits the wide exchange of enciphered information without requiring standardization of the cipher algorithms, and which makes possible the continued use of already available ciphering devices.